1. Field of the Invention
This invention relates to an image forming apparatus and an image forming method for performing image forming such as digital color copying machines, color laser printers, and ink-jet printers in which a coloring material such as toner, ink, or the like is used.
2. Description of the Related Art
In the field of image forming apparatuses using an electrophotographic method or an ink-jet method, there have been proposed image forming apparatuses for forming an image on a recording medium such as paper or an OHP sheet using dark and light coloring materials. In such image forming apparatuses, the quality of images formed on the recording medium greatly varies depending on how the density of the light coloring material is set.
In general, when the density of the light coloring material is reduced, although granularity of gradation expressed using only the light coloring material is improved, a range of gradation expressed using only the light coloring material is narrowed. Also, a difference of density between the dark and light coloring materials is increased, so that the granularity of gradation expressed using both of the dark and light coloring materials is deteriorated.
On the other hand, when the density of the light coloring material is increased, the range of gradation expressed using only the light coloring material is increased. Further, by reducing the difference of density between the dark and light coloring materials, although the granularity of gradation expressed using both of the dark and light coloring materials is improved, the granularity of gradation expressed using only the light coloring material is deteriorated in comparison with the case where the density of the light coloring material is low.
In other words, in order to obtain an image with a good granularity in an entire range of gradation, it is necessary to set the density of the light coloring material to be optimal. In practice, the density of the coloring material is preferably set to be close to that of an output image. In this case, the good granularity refers to a status of an image recorded on a recording medium where surface roughness of dots is difficult to recognize.
Conventionally, in order to solve such a problem, there have been known several techniques for securing granularity in a highlight of an image and achieving a dynamic range in a shadow of the image (see Japanese Patent No. 3391240, Japanese Laid-Open Patent Application No. 2002-139867, and Japanese Laid-Open Patent Application No. 2001-318499).
Japanese Patent No. 3391240 discloses a technique for securing granularity in the highlight of the image and achieving the dynamic range in the shadow of the image by using more than two types of ink with a different density.
On the other hand, there are substantially two types of light toner. One is a non-transparent type in which a white agent such as titanium oxide is mixed (see Japanese Laid-Open Patent Application No. 2002-139867, for example), and the other is a transparent type in which pigment is diluted (see Japanese Laid-Open Patent Application No. 2001-318499, for example).
The non-transparent type light toner is characteristic in that density thereof is saturated when an amount of attachment is increased. Japanese Laid-Open Patent Application No. 2002-139867 discloses a grounding effect, as an example, for concealing a status of a recording medium such as color, surface smoothness, and the like based on the above-mentioned characteristics. In accordance with this, granularity in a highlight of an image is obtained and a dynamic range in a shadow of the image is achieved by disposing the light toner with a low transparency at a bottom layer.
However, taking into consideration the grounding effect, it is preferable to have a high area ratio of the light coloring material and a low density of the coloring material. However, if the density of the light coloring material is low, the quality of an image is degraded as mentioned above. Further, as the density of an output image is increased, the density of the light coloring material is saturated at an early stage, so that gradation may be discontinuous at the beginning of a coloring agent of K (black) in image forming.